The present invention relates to a magnetic resonance apparatus that acquires data of a region at which magnetization transfer of protons occurs, and a method using thereof.
There has heretofore been known a CEST (Chemical Exchange Saturation Transfer) method for detecting a phenomenon of magnetization transfer of protons (refer to Japanese Unexamined Patent Application Publication No. 2012-513239).
FIG. 18 is a diagram showing an example of a sequence used in a CEST method.
In the sequence shown in FIG. 18, preparation pulses are repeatedly executed. Each of the preparation pulses includes an RF pulse X and a killer gradient pulse K for achieving a steady state of vertical magnetization. After the preparation pulses are executed and an nth preparation pulse is executed, a data acquisition sequence DAQ for acquiring data is executed by a single shot method. A z-spectrum indicative of a relationship between the frequency and a signal value is generated based on the data obtained by executing the data acquisition sequence DAQ. FIG. 19 schematically shows an example of a z-spectrum obtained when the white of a raw egg is used as a phantom. FIG. 19 is a z-spectrum obtained when a flip angle α of an RF pulse X is set to α=90°. The horizontal axis of the z-spectrum indicates the frequency, and the vertical axis thereof indicates the signal value. Incidentally, the frequency of the horizontal axis of the z-spectrum follows the concept of MR spectroscopy. The high frequency side is shown in the left, and the low frequency side is shown in the right.
In the sequence shown in FIG. 18, the RF pulse X is set to have a shape like a Gaussian distribution or a Blackman filter (trigonometric function+constant) in order to reduce side lobes of the z-spectrum. It is understood that when the z-spectrum of FIG. 19 is seen, a single signal reduction peak appears and a side lobe is sufficiently reduced. Further, since the z-spectrum causes a shift in center frequency under the influence of ununiformity in static magnetic field, it is necessary to correct the shift in its center frequency. If, however, the single signal reduction peak appears as shown in FIG. 19, the shift in the center frequency can also be easily corrected.
There is however a case where in the z-spectrum of FIG. 19, the signal reduction peak hardly appears in a proton's frequency fcest at which one desires to see a CEST effect, and hence the CEST effect is hard to be recognized. As a method for coping with such a problem, there is considered that the flip angle α of each RF pulse is set large (refer to FIG. 20).
FIG. 20 shows a z-spectrum obtained when the flip angle α of the RF pulse X is set to α=360°.
In FIG. 20, a certain degree of signal reduction is seen in the position of the proton's frequency fcest at which one desires to detect or look at the CEST effect because the flip angle α of the RF pulse X is set to α=360°. It is thus possible to recognize the signal reduction due to the CEST effect. When, however, the flip angle α of the RF pulse X is increased, side lobes of the z-spectrum are conspicuous so that a plurality of signal reduction peaks appear in the vicinity of the center frequency. Thus, since it is hard to find out the position of the center frequency when the signal reduction peaks appear, a problem arises in that it is hard to correct a shift in the center frequency.
Accordingly, there is a demand for a technology for obtaining a z-spectrum in which a signal reduction due to a CEST effect can sufficiently be confirmed and side lobes are reduced.